The background to the invention is a desire to be able to utilise organic waste as raw material in biogas plants. The organic particles manufactured by means of the machine are often used as biogas substrate. Requirements are placed on the size of the particle and in some cases, furthermore, there will also be a requirement for the substrate to be sanitised (preferably sterilised) before being used as substrate for manufacturing biogas.
Several types of process exist today for manufacturing biogas substrate from waste. Two principles which may be mentioned here are the pulping technique and the milling process.
The pulping technique may be employed on different scales and it can also handle heavily polluted waste. It resembles a mixer in its mode of operation. The waste is combined with liquid and mixed in a tank. Low-weight waste such as plastic will float up and then be removed. Heavier waste such as metal etc. will sink to the bottom. The organic waste will stay in the middle of the tank where it is shredded by the knives. This technique is expensive and has a low dry solids value since a lot of liquid is added during the process. The term dry solids value refers to how great a portion of the substrate has energy content. The low dry solids value means that the pulping plant should be located near the biogas plant in order to avoid major transport costs.
A conventional milling machine has a feed screw that feeds the raw material through a set of knives and a screen. The raw material is forced through a rotating set of knives which shreds the raw material into a fine mass which is then pressed out through a screen. By having the correct diameter on the holes of the screen, it is possible to prevent excessively large particles from being admitted to the finished product. The milling process requires finely-screened raw material. The addition of large particles and non-organic material such as metal or glass in a milling process will lead to shut-down or machine breakdown.
From the patent literature several publications are known presenting examples of the use of pulping and milling technology in industrial processes in order to produce biogas substrate.
In WO 2005061114 a plant is described for treating moist organic waste. The plant is composed of several stations where the organic waste is first treated in a mill which coarsely shreds both the organic waste and the plastics into a specific particle size. A resilient counter-steel is employed in order to avoid the mill being destroyed if the waste should contain metal. After the mill, the milled material is transferred to a unit which dissolves the particles into smaller particles. This dissolving process is implemented by adding liquid to the particles in a conical container, at the bottom of which is a tool with high rotational speed and saw blade-shaped teeth. In this process (the pulping process) the mixture's organic waste is ground into smaller particles and the plastic is ground into slightly larger particles, while heavy objects drop to the bottom of the container. With the exception of the heavy objects, the mixture is transferred to the next station where the plastic is screened out by means of a screw separator.
In WO 03092922, moreover, a plant is described where it is necessary to add liquid in order to obtain a satisfactory process. In the plant the process is also conducted in stages at several stations. In a first stage moisture is added and mixed with the waste, in the next stage the moist waste is heated until the moisture is converted to hot steam, thus causing the waste to be boiled and pulp obtained with low moisture content. The process described in WO 03092922 is therefore a pulping process.